The present invention relates to a method of manufacturing a nitride semiconductor substrate for use in a visible light emitting diode or a blue violet laser.
Group III-V nitride semiconductor such as gallium nitride (GaN), indium nitride (InN) and aluminum nitride (AlN) is preferably used as a compound semiconductor material used for a blue or green light emitting diode (LED), a blue semiconductor laser or a high speed transistor device capable of operating at a high temperature.
There is for example a well-known insulating substrate of sapphire to grow nitride semiconductor thereon as disclosed by Japanese Patent No. 3091593.
However, it is known that if a layer of nitride semiconductor is grown on a substrate of a material having a different composition from the nitride semiconductor such as sapphire, the difference between the thermal expansion coefficients of the nitride semiconductor to grow and the substrate thereunder causes the substrate to bow or have cracks. This degrades the crystallinity of the nitride semiconductor.
In recent years, there have been attempts to solve the problem related to the difference in the materials of the substrate and the layer grown thereon by forming the substrate with nitride semiconductor and forming an element structure of the same kind of nitride semiconductor thereon.
According to one such method of manufacturing a nitride semiconductor substrate, a nitride semiconductor layer is grown to have a relatively large thickness on a substrate to be a base member (base substrate), and a laser beam is irradiated on the interface between the grown nitride semiconductor layer and the base substrate. According to the proposed method, the nitride semiconductor layer irradiated with the laser beam is locally heated to be sublimed, and separated from the base substrate, so that a nitride semiconductor substrate may be provided from the nitride semiconductor layer.
According to the conventional method of manufacturing the nitride semiconductor substrate, however, only the interface being irradiated with the laser beam between the nitride semiconductor layer and the base substrate is separated, while the other part remains connected. In this case, stress concentrates at the connected part of the nitride semiconductor layer and the base substrate, and cracks are generated in the nitride semiconductor layer. As a result, it would be difficult to manufacture the nitride semiconductor substrate with a high yield by irradiation of a laser beam at about a room temperature.
In order to avoid the disadvantage, there is a known method of irradiating a laser beam by raising a substrate temperature, but in the method, raising and lowering the substrate temperature takes much time, which is not highly productive either.
At the time of growing nitride semiconductor on a base substrate, threading defects caused by lattice mismatch are introduced into the nitride semiconductor, and a resulting nitride semiconductor substrate has a high defect density.
In addition, since the laser beam is condensed into a small beam diameter, the laser beam must be efficiently irradiated in order to separate the entire joined surfaces of the base material substrate and the nitride semiconductor layer. In order to sublime the nitride semiconductor, for example, the optical density of the laser beam should be about 0.1 J/cm2 or higher, and the laser beam diameter is reduced in order to obtain the optical density. As a result, the beam diameter is small for the area of the substrate, and therefore the laser beam must be irradiated while the entire surface of the nitride semiconductor layer is scanned, which impairs the productivity from improving.